The term "membrane" is used to designate a porous structure having a surface layer in which the diameter of the pores is well-defined and determines the separating power of the membrane. Such a membrane is frequently formed by means of a macroporous support having one or more microporous layers superposed thereon.
The membrane may be constituted by a sintered ceramic (with or without the help of a vitreous phase), of microporous carbon, of sintered metal, of microporous glass, or of an organic polymer.
For a membrane constituted by a plurality of superposed layers, it is generally the surface layer that has the smallest-diameter pores and that must provide the filtering function.
It has been observed that the operating performance of such membranes depends not only on the pore diameter in its surface layer, but also on chemical or physico-chemical interactions between the surfaces of the pores and the fluids to be filtered. It is therefore essential to adapt the nature of said surfaces to the fluid under consideration.
Thus, European patent EP-0 263 468 describes a membrane constituted by a porous structure in which each grain is covered by a very thin film of (simple or complex) metal oxide. The oxide layer serves to change the surface charges of the particles constituting the membrane.
French patent FR-2 611 527 relates to inorganic membranes modified by adsorption of hydrophilic polymers on the surface of the membrane. Such modification consists in physical adsorption of the polymers and must therefore be performed before the membrane is used for any filtering operation, and after any washing operation.
U.S. Pat. No. 4,983,566 describes surface deposition of an inorganic powder that adsorbs a monolayer of perfluoride organic acid or of a perfluoride phosphonic acid for the purpose of increasing the chemical stability of the membrane over a wide pH range.
It is also known that permeating flows are maximized when the liquid to be filtered possesses properties close to the surface properties of the pores in the membrane. Thus, for a hydrophobic liquid (such as an organic solvent), maximum flow rates are obtained with membranes that are hydrophobic. There is therefore a need to have hydrophobic membranes available, in particular in the following applications:
filtering organic compounds (e.g. purifying or eliminating solid particles from oils or hydrocarbons); and PA1 filtering a fluid from which it is desired to recover a hydrophobic component (e.g. recovering oil micelles from an emulsion of oil in water) or to eliminate the hydrophilic component (e.g. eliminating water from fermenting gases). PA1 hydrogen and the following radicals: methoxy; phenoxy; methyl; ethyl; propyl and isomers thereof; butyl and isomers thereof; vinyl; allyl; butenyl; trichloromethyl; and trifluoromethyl. PA1 preparing a solution of polyphosphazene using at least one solvent of said polymer; PA1 impregnating said porous structure with said solution, thereby causing the pores of said structure to be filled with said solution; and PA1 drying substantially at ambient temperature. PA1 alcohols, e.g. methanol or propanol; PA1 cetones, e.g. acetone or methyl ethyl cetone; PA1 esters, e.g. ethyl acetate; PA1 ethers, e.g. tetrahydrofuran or diethyl ether; and PA1 aromatic hydrocarbons, e.g. benzene, toluene, and xylene.
Membranes having hydrophobic surfaces are already in existence. However, they are solely organic membranes whose characteristics do not enable them to be used under all conditions of temperature, pH, filter medium, or pressure.
An object of the present invention is to provide simply and cheaply a membrane having a surface that is irreversibly hydrophobic and that can be used without taking particular precautions.